The present invention relates generally to fluid ejection devices, and more particularly to a power switching transistor for a fluid ejection device.
One type of conventional fluid ejection system is an inkjet printing system which includes a printhead, an ink supply which supplies liquid ink to the printhead, and an electronic controller which controls the printhead. The printhead ejects ink drops through a plurality of orifices or nozzles and toward a print medium, such as a sheet of paper, so as to print onto the print medium. Typically, the orifices are arranged in one or more arrays such that properly sequenced ejection of ink from the orifices causes characters or other images to be printed upon the print medium as the printhead and the print medium are moved relative to each other.
Typically, the printhead ejects the ink drops through the nozzles by rapidly heating a small volume of ink located in vaporization chambers with small electric heaters, such as thin film resistors. Heating the ink causes the ink to vaporize and be ejected from the nozzles. Typically, for one dot of ink, a remote printhead controller typically located as part of the processing electronics of a printer, controls activation of an electrical current from a power supply external to the printhead. The electrical current is passed through a selected thin film resistor to heat the ink in a corresponding selected vaporization chamber.
In one type of printhead, a power switching device, such as a field effect transistor (FET), is coupled to each thin film resistor to control the application of the electrical current through the thin film resistors. Power is supplied to the thin film resistors via a power supply, which is included as part of the inkjet printing system.
The sales market for inkjet printing systems is very competitive. As such, to be successful, manufacturers must price these systems as low as possible and must use the lowest cost components available. If the FETs could operate at higher voltages, lower cost and lower current drive power supplies could be used in combination with higher resistance thin film resistors. This is because to heat the ink, FETs supply power to the thin film resistors where power consumed by the thin film resistors is equal to I2R or VI, where I is the current through the thin film resistor, R is the value of the thin film resistor and V is the operating voltage supplied to the FET. The turn-on energy of a printhead is herein defined to be the amount of energy that is sufficient to cause drop ejection from nozzles of the print head and is equal to power multiplied by time, where time is measured as the pulse width of a fire pulse employed for controlling the timing of the activation of electrical current through the thin film resistor. Thus, higher operating voltages and/or higher resistance thin film resistors reduce the current required to reach the turn-on energy of the printhead.
The FETs are typically obtained from semiconductor manufacturers using well established semiconductor processes in order to achieve the lowest possible unit prices. Increasing the operating voltage levels for the FETs typically requires additional processing steps. Because the unit price of the FETs must be competitive at high manufacturing volumes, variations in semiconductor processes such as design rule or process step changes are avoided.
For reasons stated above and for other reasons presented in greater detail in the Detailed Description section of the present specification, an approach is desired which will enable the manufacturers to use low cost power supplies by improving the operating voltage level of the FETs.
One aspect of the present invention provides a method of manufacturing a power switching transistor for a fluid ejection device. The method includes forming a diffused drain region and a diffused source region separated by a channel region. The method further includes doping the diffused drain region and the diffused source region with a first dopant. Further, the method includes doping a first portion of the diffused drain region with a second dopant such that the first portion of the diffused drain region has a greater impurity concentration than at least the second portion of the diffused drain region and has a greater impurity concentration than the diffused source region.